DD-WRT has a licensing relationship with Broadcom and are believed to have direct access to closed source code, which they can do because of their licensing model. Tomato does not, and is thus restricted to whatever binary driver blobs Broadcom ships commercially for other routers.

The world has access to some sources only - the same as included with any other Broadcom SDK in any other GPL package. The binary blobs are still there for the core part of the driver.

Yes, theoretically it's possible to port the binary driver from DD-WRT to Tomato. But there's no point… DD-WRT made it as difficult as it could possibly get by shuffling around members in core kernel structures etc, basically making their particular build of the driver compatible only with pretty much messed up DD-WRT version of the kernel. Anyway, there will be other GPL packages with newer driver available (there are some already) - and eventually the driver in Tomato will be updated too to support newer chipsets.